Reads get worse when the battery is low
Usually a power saving setting rather than a weaker field. Several devices restrict or switch off NFC under battery saving modes, which is device behaviour rather than a property of the technology. Charge the phone, retest, and check the battery settings.
A setting, not physics
Battery saving modes reduce what a device does in the background, and on some phones that includes switching NFC off or restricting when it looks for tags.
Where that happens, the symptom is not a shorter read distance. It is no reading at all, or reading only while an application is open, which people describe as reads getting worse.
The behaviour varies between manufacturers and versions, so a rule that holds on one device may not on another.
Android's power management documentation lists what Doze restricts: network access, wake locks, alarms, Wi-Fi scans, sync adapters and scheduled jobs. NFC is not on that list. So where a device does cut NFC under a saving mode, it comes from the manufacturer layer rather than from the platform behaviour Google documents, which is why testing the phone in front of you settles it and a general rule does not.
Testing it properly
Charge the phone above half and repeat the exact test that failed. If it works, the battery state was the variable.
Then turn the power saving mode on deliberately, with the battery still charged, and test again. If it fails, the mode is the cause rather than the charge level.
That second test is the one people skip, and it separates a setting from a hardware effect.
Keep everything else identical: same tag, same spot, same case, same duration.
What to check on the phone
Whether a battery saving mode is active, including any automatic mode that switches on below a threshold.
Whether NFC is still enabled in settings, since some modes turn it off and leave it off.
Whether the phone is simply too low to behave normally at all. A device close to shutting down restricts a great deal, and NFC is not the interesting part of that.
And whether anything else changed at the same time, since a phone left to run flat has often also been in a pocket, under a case, or updated.
What it means for a deployment
You cannot control the battery state of the phones that will tap your tags. Some proportion of visitors will be in a power saving mode.
The practical response is margin. A larger tag and a good mounting surface leave room for a phone that is not at its best.
And a visible instruction helps, because a person whose phone is in a saving mode may need to open an application rather than expecting a background read.
Do not build a flow that assumes background reading. It is the first thing power saving modes take away.
Frequently asked questions
Does a low battery weaken the NFC field?
The commonly observed cause is a power saving setting rather than a weaker field. Test with the mode on and the battery charged to tell them apart.
Will NFC work if my phone is switched off?
Tag reading needs the phone powered, so a phone that has shut down reads nothing. Payment features on some devices behave differently and are a separate mechanism.
Why does it work again after charging?
Because the power saving mode switched off at a threshold. That is a setting changing, not the hardware recovering.
Can I stop battery saver disabling NFC?
Where the device exposes it as a setting, yes. The behaviour differs between manufacturers, so check that specific phone.